There is known a post-processing apparatus, which performs post-processing on sheets transported from an image-forming apparatus. The post-processing apparatus includes a processing tray and a standby tray. In the processing tray, post-processing is performed. The standby tray is provided above the processing tray. During the post-processing performed on sheets in the processing tray, the standby tray temporarily retains subsequent sheets. When the processing tray becomes empty, the standby tray drops the retained sheets toward the processing tray. In the post-processing apparatus, it is desirable that sheets be stably moved from the standby tray to the processing tray. When the post-processing apparatus is provided with a mechanism to stably move sheets from the standby tray to the processing tray, however, increase in size of the post-processing apparatus may be caused.